function Y_ECHO = audio_echo(Y, FS, AMPLITUDE, DELAY)
  
  % length of input signal
  [len_y, len_track] = size(Y)
  
  % length of audio
  len_seconds = len_y / FS;
  % length of delay time
  len_delay = ceil(FS * DELAY / 1000);

  % extand the input signal
  Y_extend = [Y; zeros(len_delay, len_track)];
  % delay signal to generate the echo
  Y_delay = [zeros(len_delay, len_track); AMPLITUDE * Y];

  % >> signal with the echo
  Y_ECHO = Y_extend + Y_delay;

end
